Currently, fluid enclosures are designed and built independently of the fluid being stored, or of any storage material that would be inserted within the enclosure. In its simplest form, a conventional pressure vessel can be used to contain a fluid, such as a compressed gas or a liquefied gas. The pressure vessel must be designed to accommodate the maximum pressure of the fluid without failure. Such simple design approaches can be extended to incorporate a storage material by filling the pressure vessel with storage material. In this case, the pressure vessel must now withstand the fluid pressure, as well as the stress induced by the force of the storage material exerted on the internal pressure vessel walls. Presently, these vessels tend to be of a cylindrical shape.
When very small storage systems are required, or when irregular (i.e. non-cylindrical) shapes are called for, the overall approach of employing conventional pressure vessels becomes problematic. In order to contain the internal pressures and mechanical stresses induced by a storage material, wall thickness and material properties of the enclosure must be sufficient to prevent rupture. Material properties considered include tensile strength, ductility, material compatibility, enclosure geometry, stress factors, etc. As a result, the range of materials that can be used to construct the enclosure is limited, and only vessel geometries which do not overly amplify the internal pressures as enclosure stress can be considered.
Challenges to fluid enclosure design are amplified when incorporated in small systems, such as in a small or micro scale fuel cell. In small systems, fluid enclosure wall thickness consumes a significant portion of the volume of the enclosure. Prismatic shapes or irregular form factors are very difficult to utilize since they will bow outward under even modest fluid pressure. When absorbing materials (e.g. hydrides) are used, the mechanical strain on the internal tank walls can induce large stresses.